1. Field of the Invention
The invention concerns a process for the preparation of low temperature flexible polyurethane elastomers by reacting
(a) organic polyisocyanates, preferably polyester polyol prepolymers of organic polyisocyanates with NCO contents of 10 to 25 percent by weight based on the weight of polyisocyanate, PA1 (b) polyester diols with OH numbers of 40 to 120 and glass transition temperatures of -10.degree. to -80.degree. C. or hydroxyl group containing polytetrahydrofurans, PA1 (c) alkoxylated esters of fatty acids and/or alkoxylated terpene alcohols, PA1 (d) chain extenders or cross-linking agents in the presence of catalysts and optionally blowing agents in quantities of 0.1 to 6 parts by weight of component (c) per 100 parts by weight of component (b) and optionally (d) wherein the mole ratios of reactive hydrogen atoms of components (b) and (c) and optionally (d) to NCO groups of component (a) range from 0.90:1 to 1.15:1.
2. Prior Art
The preparation of polyurethane elastomers from polyester polyols, polyisocyanates and chain extenders is described in numerous literature sources, for example, Journal of Cellular Plastics 13, 303 (1977), Journal Cellular Plastics 10, 283 (1974), and Polymers Paint Color Journal, Polyurethanes Symposium, University Surrey, Sept. 23/25, 1974 and in British Pat. No. 1,485,986.
In addition to the many physical properties of these polyester urethane elastomers such as mechanical stability values, elasticity, elongation, low compression set, light fastness, oxidation and hydrolysis resistance, a high degree of low temperature flexibility is particularly desired.
These properties are affected primarily by the choice of raw materials for these polyurethane elastomers. In practice, linear polyesters are preferably used as the polyol component having terminal hydroxyl groups which are produced by the condensation of dicarboxylic acids with diols or products derived from caprolactones. Compared to polyurethane elastomers based on polyether polyols, these polyester polyurethanes are characterized by a greater physical strength and, furthermore, by a lesser susceptibility to the effects of light and oxidation.
While polyester polyols based on, for instance, adipic acid/1,6-hexanediol or adipic acid/1,4-butanediol have a low glass transition temperature and thus good stability when exposed to cold in the amorphous state in the polyurethane elastomer there is an increased tendency to crystallize which results in a poorer low temperature flexibility.
In addition to this, the tendency toward crystallization in a polyester polyol component also makes the processing more difficult when using the two-component one-shot system and the commonly used low and high pressure foaming equipment.
Diol mixtures were employed for the preparation of polyester polyols in attempts to prevent the crystallization and to improve the low temperature flexibility properties of the polyurethane elastomers.
The tendency to crystallize may also be reduced by incorporating branched diols in the polyester polyol, particularly 2,2-dimethylpropanediol-1,3 in addition to 1,6-hexanediol. In spite of this, the polyurethane elastomers produced with these polyester polyols did not display sufficient improvement in their low temperature flexibility. This is particularly true for cellular polyurethane elastomers with hardnesses of approximately 45 to 75 shore A and densities of approximately 0.35 to 0.65 grams per cubic centimeter when they are used for shoe sole applications, where temperatures of -5.degree. C. to -40.degree. C. are not unusual in countries with prolonged periods of sub-zero temperatures. The requirements for high quality shoe soles of polyurethane elastomers are correspondingly high and fatigue bending tests are conducted at -25.degree. C. (for instance, SATRA Test Method PM.60 in accordance with ASTM D-1052-55 and also RAL-RG 702/1 in accordance with DIN 53522). These tests are passed by the prior art polyurethane elastomers which can be used for shoe soles at room temperatures but not at test temperatures of -25.degree. C.
The purpose of this invention was to develop polyurethane elastomers preferably cellular which have low temperature flexibility and which have good flexural strength at -25.degree. C. in order that they may be used as shoe soles in countries with prolonged sub-zero degree temperatures.